As a result of the increased emphasis by Federal and State governments in combating air pollution, the coatings industry is expending considerable effort in eliminating or at least substantially minimizing the emission of solvent vapors to the atmosphere from coating compositions. As a part of this effort, the coatings industry has launched a major effort to develop aqueous or water-based coating compositions in which organic solvents have been completely eliminated or in which the organic solvents constitute only a very minor proportion of the total liquid medium.
In view of the excellent properties of solvent-based epoxy coating compositions for various coating applications, those in the coatings art have been extremely interested in developing aqueous coating compositions derived from epoxy resins. Prior efforts to develop such compositions involved reacting hydroxy carboxylic acids and epoxy compounds. However, in reacting such compounds, two types of reaction may result due to the chemical nature of the materials used. The hydroxyl group of the hydroxy acid may react with the epoxide groups to form ether linkages, or the carboxyl group or groups of the acid may react with the epoxide to form ester groups. Both reactions may occur in an uncontrolled reaction to yield products having mixed ether or ester linkages to a non-predetermined degree. Such reaction with the epoxides and acids previously employed have not been tolerable since the ultimate products have not generally been suitable for any practical purpose.
In an attempt to overcome these problems, the art has attempted to optimize the etherification portion of the reaction while minimizing the esterification portion of the reaction (See, e.g., U.S. Pat. Nos. 3,404,018 and 3,410,773). Similarly, the art has attempted to utilize products containing both ester and ether linkages (See, e.g., U.S. Pat. Nos. 3,707,526 and 3,792,112). The compositions produced using the above techniques suffer from certain disadvantages, including poor cured film saponification resistance, low hydrolysis resistance, and lack of adequate package stability.